Skip to main content

Health condition · Clinically reviewed

OHSS, the fertility-treatment complication worth knowing about before you start.

Most cases are mild and settle at home. A smaller number need hospital care - knowing the difference, and the warning signs, matters.

Jump to treatment
A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against the RCOG Green-top Guideline and peer-reviewed sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK fertility-unit practice including antagonist protocols and thromboprophylaxis.

Key facts

OHSS at a glance.

The essentials, in plain English - what it is, who it affects, and how it’s classified and managed in the UK today.

  • What it is

    An exaggerated ovarian response to fertility hormones causing enlarged ovaries and fluid shift into the abdomen and chest.

  • When it happens

    A complication of ovarian stimulation for IVF or ovulation induction, usually within days of the trigger injection or embryo transfer.

  • Who is at risk

    Women with PCOS, a high egg yield, or a strong hormonal response are at significantly higher risk.

  • Severity bands

    Classified as mild, moderate, severe or critical - this classification decides where and how you are monitored.

  • Serious risks

    Severe disease brings a genuinely increased risk of venous thromboembolism, kidney impairment and breathing difficulty.

  • Prevention

    GnRH antagonist protocols, coasting and cycle cancellation can all reduce OHSS risk in future cycles.

Why this guide matters

Knowing the warning signs changes outcomes.

OHSS is common in a mild form and rare in a severe one - but early recognition is what keeps it that way. The three points below shape everything else on this page.

  • It follows ovarian stimulation

    OHSS develops after the hormone injections used for IVF or ovulation induction, usually within a week of the trigger shot.

  • Severity decides the setting

    Mild disease is managed at home; moderate-to-severe disease needs hospital admission and closer monitoring.

  • Prevention is often possible

    If you have had OHSS before, an antagonist protocol or a freeze-all strategy can significantly cut the risk next time.

How the diagnosis is made

From first symptoms to a clear plan.

The steps a UK fertility unit will normally follow, based on the RCOG Green-top Guideline - so you know what to expect and why.

  1. 01

    Assessing

    Clinical symptom review

    A structured check of bloating, abdominal discomfort, nausea, breathlessness and how quickly symptoms are progressing.

  2. 02

    Assessing

    Risk-factor check

    PCOS, a high antral follicle count, a high oestrogen level and a high number of eggs collected all raise suspicion.

  3. 03

    Confirming

    Ultrasound assessment

    Measures ovarian size and looks for free fluid (ascites) in the abdomen - both feed into the RCOG severity classification.

  4. 04

    Confirming

    Bloods for haemoconcentration

    Haematocrit is the key marker of fluid shift out of the bloodstream, alongside electrolytes, renal and liver function.

  5. 05

    Confirming

    Severity classification

    Findings are combined into mild, moderate, severe or critical OHSS - this decides outpatient care versus admission.

  6. 06

    Preparing

    Fluid balance and VTE review

    Strict fluid balance monitoring begins, alongside an assessment of thromboembolism risk in moderate-to-severe disease.

  7. 07

    Preparing

    Specialist unit involvement

    Moderate-to-severe or critical OHSS is managed jointly with a specialist commissioned fertility or reproductive medicine unit.

Typical timeline: symptoms build over the days after trigger, with severity peaking within one to two weeks.

Symptoms

What OHSS actually feels like.

Mild symptoms are common after stimulation. The features below help separate a normal response from one that needs closer attention.

  • Bloating and distension

    Increasing abdominal swelling and a feeling of tightness are usually the earliest symptoms after stimulation.

  • Abdominal discomfort

    Mild aching to more marked pain across the lower abdomen as the ovaries enlarge and fluid accumulates.

  • Nausea and vomiting

    Common in mild disease, but persistent or severe vomiting is a sign of a more significant case.

  • Ascites on ultrasound

    Free fluid in the abdomen visible on scan - a defining feature of moderate and severe OHSS.

  • Breathlessness

    Suggests a pleural effusion - fluid around the lungs - and is a marker of severe disease needing urgent review.

  • Reduced urine output

    Oliguria reflects fluid shifting out of the circulation and into the abdomen, and needs close fluid balance monitoring.

  • Leg swelling or calf pain

    A possible sign of venous thromboembolism, a serious potential complication of severe OHSS.

  • Red flag - rapid deterioration

    Fast-worsening breathlessness, pain or reduced urine output warrants same-day specialist assessment.

Treatment

How OHSS is managed in the UK.

Outpatient care for mild disease, hospital-based fluid and clot management for moderate-to-severe cases, and prevention strategies for next time.

  • Outpatient monitoring

    Mild OHSS is usually managed at home with oral fluids, analgesia and clear safety-netting on symptoms to watch for.

  • Oral fluids and analgesia

    Simple pain relief and steady oral fluid intake support most mild-to-moderate cases while the body settles.

  • Hospital admission

    Moderate-to-severe OHSS needs admission for close monitoring of fluid balance, bloods and symptoms.

  • IV fluid management

    Careful intravenous fluid replacement corrects haemoconcentration without tipping into fluid overload.

  • Thromboprophylaxis

    Given the significantly increased VTE risk in moderate-to-severe OHSS, preventive anticoagulation is routine.

  • Paracentesis

    Draining ascitic fluid relieves pressure, breathlessness and discomfort in severe, symptomatic cases.

  • Critical care admission

    Reserved for critical OHSS with major fluid shifts, breathing difficulty or organ impairment.

  • Future-cycle prevention

    GnRH antagonist protocols, coasting or cycle cancellation reduce the risk of OHSS recurring in later treatment.

What this guide is based on

The sources behind every claim on this page.

UK national guidance and specialist society standards, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your fertility unit knows your treatment history and can tell you which parts apply to you. If in doubt, get seen.

  • Royal College of Obstetricians and Gynaecologists (RCOG). Green-top Guideline: The Management of Ovarian Hyperstimulation Syndrome.

  • Human Fertilisation and Embryology Authority (HFEA). Guidance on OHSS and fertility treatment safety.

  • European Society of Human Reproduction and Embryology (ESHRE). Guideline on ovarian stimulation for IVF/ICSI.

  • NICE. Fertility problems: assessment and treatment (CG156).

Red flags

When OHSS needs urgent attention.

Most OHSS is mild and self-limiting. These are the situations that aren’t - and where same-day or emergency care is needed.

  • Severe breathlessness

    Suggests a significant pleural effusion or, rarely, a pulmonary embolism - needs same-day emergency assessment.

  • Chest pain or calf swelling

    Possible venous thromboembolism - a serious potential complication of severe OHSS requiring urgent review.

  • Marked reduction in urine output

    A sign of significant fluid shift and possible kidney impairment - needs urgent fluid balance review.

  • Rapidly increasing abdominal girth

    Fast-accumulating ascites can signal progression from moderate to severe disease within hours.

  • Persistent vomiting or inability to keep fluids down

    Risks worsening haemoconcentration and dehydration - needs same-day assessment.

  • Dizziness or fainting

    Can reflect reduced circulating volume from fluid shifting into the abdomen - seek urgent advice.

  • Fever

    Raises the possibility of infection or ovarian torsion and should always be reviewed promptly.

  • Severe, one-sided abdominal pain

    Can indicate ovarian torsion, a surgical emergency that can occur alongside enlarged, OHSS-affected ovaries.

  • Critical OHSS features

    Tense ascites, significant breathing difficulty or oliguria together need immediate specialist and often critical care input.

Living with it

A short illness, usually with a clear end point.

Four things that make the biggest difference while OHSS runs its course - watching your weight, balancing fluids, staying gently mobile and knowing when to call.

A quiet reminder

Most cases resolve within one to two weeks.

If a pregnancy results from the cycle, symptoms can last a little longer as rising hormone levels keep the ovaries active.

  1. 01 Monitor

    Weigh yourself daily

    A steady daily weight and waist measurement helps you and your team spot fluid build-up early.

  2. 02 Fluids

    Balance, don’t overload

    Steady oral fluids are usually recommended, but more is not automatically better - follow your unit’s specific advice.

  3. 03 Move

    Gentle movement, not bed rest

    Light activity where possible helps reduce clot risk - full bed rest is generally discouraged unless advised otherwise.

  4. 04 Contact

    Know when to call

    Keep your fertility unit’s out-of-hours number to hand and use it early if symptoms are progressing.

Frequently asked

Everything we get asked about OHSS.

Quick answers on risk factors, severity grading, blood clot risk and preventing OHSS next cycle.

  • What is ovarian hyperstimulation syndrome?

    OHSS is a complication of fertility treatment in which the ovaries respond excessively to stimulation hormones, becoming enlarged and causing fluid to shift out of the bloodstream into the abdominal and sometimes chest cavity. It ranges from mild bloating to a severe, hospital-treated illness.

  • Who is most at risk of developing OHSS?

    Women with polycystic ovary syndrome, a high antral follicle count, a strong hormonal response to stimulation, or a high number of eggs collected are at significantly higher risk. Younger age and a low body mass index are also recognised risk factors.

  • How is OHSS classified and why does it matter?

    The RCOG Green-top Guideline classifies OHSS as mild, moderate, severe or critical based on symptoms, ultrasound findings and blood results. This classification decides whether you are managed at home or need hospital or critical care admission.

  • Can mild OHSS be managed at home?

    Yes - mild OHSS is usually managed as an outpatient with oral fluids, analgesia and close symptom monitoring. You will normally be given clear guidance on symptoms that mean you should contact your fertility unit urgently.

  • Why is there a blood clot risk with OHSS?

    Haemoconcentration from fluid shifting out of the bloodstream, combined with high oestrogen levels, significantly raises the risk of venous thromboembolism in moderate-to-severe OHSS. Thromboprophylaxis is a routine part of hospital management.

  • How is OHSS prevented in future fertility cycles?

    Units often switch to a GnRH antagonist protocol, use coasting (pausing stimulation), adjust trigger medication, freeze all embryos rather than transferring fresh, or in some cases cancel a cycle - all aimed at reducing the risk of OHSS recurring.

Related content

Keep reading.

Pulse Healthcare concierge

Send us your enquiry

A concierge service for UK private healthcare. We match you with the best vetted clinics and consultants in our network - they then contact you directly.

So we can match you to the right clinician close to you.

We reply to every enquiry within 24 hours (Mon–Fri). Confidential - your details are never shared outside our vetted consultant network.